Edm coolant



R. S. WEBB Aug. 15, 1961 EDM COOLANT Filed Jan. 26, 1959 if a/r/v K United States Patent 2,996,602 EDM COOLANT Robert S. Webb, Franklin, Mich., assigner to Elox Corporation of Michigan, Royal Oak, Mich., a corporation of Michigan Filed Ian. 26, 1959, Ser. No. 789,065 Claims. (Cl. 2'19-169) This invention relates to improvements in the art of electrical-discharge-machining, sometimes referred to as sparkmrachiningf electro-spark-machining, or EDM.

EDM involves the cutting or grinding of conductive workpieces by overvoltage discharges across a gap between the work-piece and an electrode in the presence of suitable coolant. Many different types of coolant have been used such as water, light oils, kerosene, etc.

The yfunction of the coolant is to flush away the particles eroded from the workpiece (and in some instances from the electrode), and to provide a dielectric medium in the discharge gap of selected strength. As an example, most light oils and kerosene used have a dielectric strength of approximately 250 volts per .001 inch. This means that for a machining gap of .001 inch, the gap power supply must have a potential of at least 250 volts. Such a high 'voltage is dangerous to the operator and is undesirable for many machining conditions because it is impossible to achieve the fine finish and dimensional control required.

With such coolant, a gap working voltage of 50 volts, which is normal for most machining, requires that the gap be reduced to two-tenths of .001 or .0002 inch. Such a minute gap causes instability in the operation of the machine because of the d-iiculty in holding such a small gap spacing with the automatic power feed equipment. In addition, the small gap space permits less coolant to circulate in the gap zone with correspondingly fewer discharges and lower cutting rate. This low machining rate is Iimportant and highly undesirable.

It -is the principal object of my invention to improve the EDM process by making it possible to machine at relatively lower voltages with relatively greater gap spacing than is now possible.

Another object is to provide improved coolants for use in EDM.

My improvement in the general sense consists in the use of an emulsified coolant, specifically a water-in-oil emulsion. A water-in-oil emulsion differs from the ordinary water-oil emulsion in that droplets of water are suspended in acontinuous oil phase, in contrast to droplets of oil suspended in water. In this instance, the oil constitutes the dielectric insulator in the gap with the water droplets forming short, disconnected, relatively conductive paths through the dielectric.

Water-in-oil emulsions occur naturally in petroleum from producing wells `and in petroleum which has been stored in storage tanks. They are sometimes referred to as out oil, roily oil, and emulsiiied oil. One commercially obtainable water-in-oil emulsion is known in the trade as Iris Fluid `902 `and is manufactured by Shell Oil Co. This is an emulsion containing about 35% water, but whether it is a rened natural product or a prepared product is not known.

Water-in-oil emulsions comprising animal or vegetable oil may be readily prepared by mixing oil and water (usually in quantities not exceeding 50% water) with an added surfactant or emulsier. Such an emulsier, for example, is known in the trade as Polytergent B200` and is obtainable commercially from Olin Mat/hieson Chemical Corporation.

Patented Aug. 15, 1961 For a better understanding of my invention, reference is made to the accompanying drawing, in which:

FIG. l is a greatly enlarged schematic showing of an EDM gap through ywhich a water-in-oil emulsion is being circulated; and

FIG. 2 is a graphical representation of discharge across the gap.

Referring to the figures, a workpiece 10 is shown disposed in spaced relation to an electrode 12. It is assumed that some form of automatic electrode power feed is present for feeding the electrode toward the piece as erosion takes place and for maintaining selected electrodework spacing. It will be further assumed that a suitable power source is present for creating over-voltage discharges or sparkovers across the gap.

The electrode has a passageway 14 through which coolant may be pumped into the gap zone. The coolant in this instance comprises a continuous dielectric phase 26 such as, for example, transformer oil, kerosene or the like, in which are suspended droplets or globules 24 of electrolyte or duid that tends to be conductive rather than non-conductive such as water plus an ionizable substance.

The emulsion 26 spreads out uniformly between the juxtaposed areas of the electrode and workpiece in the manner shown, although it will be appreciated that the showing is diagrammatic only and the gap spacing, size of droplets, etc., is greatly enlarged for purposes of explanation. The droplets 24 of water or electrolyte to be accurate, will be fairly uniformly distributed throughout the dielectric phase 26, but will 'be discrete in nature.

When the gap fires, the resultant discharges will take place at various points over the face of the electrode in accordance with the paths of low dielectric strength caused by proximate spacing of the water droplets.

FIG. 2 illustrates diagrammatically a discharge 22 which follows a path defined by the four water droplets 24 which form a path'of relatively low dielectric strength between the electrode and workpiece even though they are entirely surrounded by the dielectric phase 26 of the emulsion. It will be readily appreciated that the dielectric of the gap is effectively reduced in length from the actual distance 18 to the sum of the distances 20 if the small dielectric strength of the electrolyte is neglected.

It is thus apparent that by using as a coolant an emulsion having a continuous dielectric phase with suspended conductive droplets, it is possible to machine with a gap several times the length which could be used with dielectric coolant alone, voltage conditions being equal. The advantages inherent in the use of dielectric coolant are thus preserved and the additional advantages flowing from wider gap spacing are made possible-yet the necessity of increasing the working gap voltage to an undesirable or unsafe level is avoided.

Another and somewhat unexpected advantage results from use of emulsified coolant. In conventional machining with dielectric coolant, successive electrical discharges tend to translate over the face of the electrode to some extent. This is caused by electrode erosion, workpiece erosion, accumulation of particles in the gap zone, etc. However, there is a tendency for immediately successive discharges to take place from a single point or an adjacent point on the electrode face until that area breaks down, then the discharges will shift to another area. With emulsified coolant, however, a succession of discharges in one area will vaporize the water droplets causing a gaseous condition in that immediate zone which results in momentary higher dielectric gradient than adjacent unvaporized zones, whereupon the discharge path will rapidly move to a remote point on the electrode face. This results in a more even distribution of heat over the faces of the electrode and workpiece and tends to eliminate failure of the electrode caused by uneven heating thereof.

An ideal emulsion -would be one containing the highest percentage of water (electrolyte) with stable characteristie and reasonable viscosity-in the order of 50 centipoises.

Two examples of water-in-oil emulsions which have been used to advantage are:

( 1) Shell Oil Company Iris Fluid 902;

(2) Animal or vegetable oil 45%, water 45%, Olin Mathieson Chemical Corporation Polytergent B200 throughly mixed.

In each case, an appropriate amount of a salt, acid or base is added to the water as a coupling agent and to increase the electrolytic action of the water.

I claim:

1. The method of electrical-discharge-machining which comprises, causing intermittent electrical-discharge across a gap between an electrode and a workpiece while maintaining in the gap a coolant consisting of a water-in-Oil emulsion.

2. The method of electrical-discharge-machining which comprises, causing intermittent electrical-discharge across a gap between an electrode and a workpiece While maintaining in the gap a coolant consisting of an emulsion having a continuous dielectric phase with droplets of ionizable uid dispersed therein.

3. The method of electrical-discharge-machining which comprises, causing intermittent electrical-discharge across a gap between an electrode and a workpiece while maintaining in the gap a coolant consisting of an emulsion comprising essentially Shell Iris Fluid 902.

4. The method of electrical-discharge-machining which comprises, causing intermittent electrical-discharge across `a gap between an electrode and a workpiece while maintaining in the gap a coolant consisting of an emulsion comprising animal or vegetable oil water 45% and Mathieson Polytergent B200 10%.

5. The method of electrical-discharge-machining which comprises, causing intermittent electrical-discharge across a gap between an electrode and a workpiece while maintaining in the gap a coolant consisting of a water-in-oil emulsion comprising animal or vegetable oil 45 water 45 and a suitable surfactant 10%.

References Cited in the file of this patent UNITED STATES PATENTS 1,861,398 Lant May 3l, 1932 2,730,602 Portereld Ian. l0, 1956 2,838,652 Porterfleld June 10, 1958 OTHER REFERENCES American Machinist, March 3, 1952, p. 139. 

1. THE METHOD OF ELECTRICAL-DISCHARGE-MACHINING WHICH COMPRISES, CAUSING INTERMITTENT ELECTRICAL-DISCHARGE ACROSS A GAP BETWEEN AN ELECTRODE AND A WORKPIECE WHILE MAINTAINING IN THE GAP A COOLANT CONSISTING OF A WATER-IN-OIL EMULSION. 